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amplifying signal from crank position sensor



Jan 1, 1970
My 1989 Jeep Cherokee started having an intermittent problem where it
dies sometimes when I'm driving and is getting harder to start..
After reading this page:
I tested the crank position sensor (cps) with a voltmeter. It showed
about .4 volts ac or a little less when cranking. That's below the
acceptable voltage range of .5 to .8 volts stated in the article.
The cps is on the bell housing, very difficult to get to. It's
recommended that you lower the transmission a little. You need a
floor jack, which I don't have.

you don't need a floor jack

jack the vehicle up a little and support the transmission, lower it
until the support it taking the weight, undo the mounts and then jack
the vehicle up until you have enough clearance (this is more work, but
will get the job done)
I got the idea of building a circuit to amplify the signal from the cps.
Some basic facts... there are only two wires coming from the cps.
With the car turned off, the cps is supposed to measure about 275 ohms
(mine measures 230).

a typical CPS input looks something like this.

+----[R]-- 1/2Vcc
| |\
| | \
=== | >---+---
| | / |
--[R]--+-+--|-/ |
| |/ |
| |

the sensor puts out a higher voltage as the engine speeds up, the
resistors on the left, and the capacitor form a low-pass filter which
compensates for that.

As I indicated before, the cps generates a small
signal (detectable with a voltmeter set on ac) when the flywheel is
turning and the slots are flying past the sensor.
While I had the connector unplugged, I also tested the pins on the
wiring harness side of the plug. With the ignition key turned on both
pins show 3.37 volts with respect to ground and zero volts across the

the v0ltage needs to increase with rpm I'm not sure how high it needs
to get though.


Tony Williams

Jan 1, 1970
kell said:
The capacitor trick worked. I tried 0.1 uF first; car started ran
bad. Bunged in a 0.01 uF and ALL'S WELL.

Assuming the VRS has an inductance of about 100mH,
0.01uF gives a resonance at about 5KHz, and with a
Q of >10 (actual value dependant on the input-R of
the differential amplifier). The response is
probably a little too peaky. The output will fall
off rapidly at higher frequencies, and might give
trouble at higher engine speeds. The response can
be flattened with a 1k to 2k in series with the cap.